Restring PV array to not exceed MPPT limit

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ant asked Aug 28, '20

Restring PV array to not exceed MPPT limit

I have a 150/60 MPPT with a string of 12 panels (Voc = 37.4 each). It's been in place for about 3 years, but about 4 or 5 times a year I get a VRM warning that the voltage is greater than 150V, which then clears after 30 seconds or so. I've never worried about this, assuming it was just a transient issue. However I've just read @Guy comments on other posts that even if it a couple of volts over 150 then I must immediately switch off the device, and reconfigure the strings. I don't know how they are currently configured, but am I right that if the voltage is maxxing out at just over 150V , then they will be in 3 parallel sets of 4 series panels. So must I change this to 4 parallel sets of 3 panels, which will bring the max voltage to 114V which is well below the 150 limit? What is the downside to doing this?

MPPT Controllers Solar Panel

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Answer 1 · 2020-08-28T19:46:56Z

wkirby answered · Aug 28, 2020 at 07:46 PM

I would reconfigure your array as soon a possible. 4 parallel sets of 3 in series will solve the problem and prevent damage to the MPPT.
The only fairly minor downside is that there is more PV current in the cable from where the arrays are paralleled to the MPPT terminals. It will have to carry ~28A instead of ~21A (finger in the air calculations based on typical PV panel specs.). If your cable is up to it already then there is no problem.
The major upside is that your MPPT won't be a risk of being damaged. You have been fortunate so far that this hasn't happened.

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ant commented · Aug 28, 2020 at 08:22 PM

Thankyou @wkirby . I'll do so. It will take a week or so to get it changed. Is there anything I can do to reduce the chance of this causing a fault in the interim? Perhaps turn off the panels before the batterys are full, or be sure to run a load when they are nearing full? (I'm assuming this happens when the sun is at its peak, battery is full, and there is briefly no load?)

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Phil Gavin ant commented · Aug 28, 2020 at 09:37 PM

The PV voltage will be too high all the time. I don't know how you have gotten away with this.

Are you sure it's one string of 12 and not 2 strings of 6?

I would suggest you disconnect until you can reconfigure.

Please measure the PV voltage once disconnected and report back.

Certainly do not disconnect and reconnect during the day.

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ant Phil Gavin commented · Aug 29, 2020 at 07:03 AM

Sorry Phil, I expressed myself badly. I have 12 panels (not a string of 12). They must be configured as 3 parallel sets of a string of 4 . As per wkirby I'll reconfigure them as 4 sets of strings of 3

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Answer 2 · 2020-08-29T00:07:59Z

trip-tracks answered · Aug 29, 2020 at 12:07 AM

The alternative to rewiring your strings might be to replace the 150/60 with a 250/60.
It would depend on how easy it is to rewire your strings vs the cost of a 250/60.

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Answer 3 · 2020-08-29T07:42:36Z

Paul B answered · Aug 29, 2020 at 07:42 AM

YES re configure the ARRAY and until you have done so TURN OFF the PV. or as Trip Says by a 250 volt unit and then sell the 150 volt unit on Ebay or similar. do the math and make a informed decision based on what best suits YOU.

I agree with the others you are lucky to not have damaged the unit already

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